2channel (2ちゃんねる, ni channeru; 2ch for short) - also known as 5channel (5ちゃんねる, go channeru; 5ch for short) since 2014 split.
A Japanese internet forum, often thought to be the largest in the world in its prime. It has over 600 boards covering a wide range of topics, as their slogan ('From "hacking" to "side dishes for tonight's dinner"') would suggest.
2ch/5ch plays a large part in Japanese internet culture and has created a culture and language in and of itself. ASCII and Shift-JIS art is popular there and many meme/neta and characters have been created based on it. In addition, several internet celebrities have been born there, as have many collaborative projects and pranks (ie: Densha Otoko, Buront, Kasane Teto and the Vippaloids.) However, not unlike 4chan of the west, 2ch/5ch is infamous for its near anarchic, non-existent moderation, which results into the site being a hotbed for political extremists and illegal activities.
In 2014, after the dispute about the domain ownership, the original 2ch.net domain was seized by a group of American shareholders and renamed as 5ch.net, wheas the original 2ch founders later opened a "successor" site based in Singapore with registered domain of 2ch.sg. Both sites were still disputing over their claim of being the "legitimate" successor of 2ch.net to this day.